A conventional fuel supply system uses a positive displacement pump, for example a gear pump, driven from the gas turbine engine trough the intermediary of an accessory gearbox, to provide fuel through a fuel metering system to the burners of the gas turbine engine. The positive displacement pump which, in the interests of clarity will be referred to herein as a “gear pump”, receives fuel from a fuel supply through a low pressure lift pump or the like, and it will be recognised that the rotational speed of the gear pump, and thus the output of the pump is directly proportional to the shaft speed of the gas turbine engine. Generally the capacity and therefore the size of the pump is calculated on the basis of the maximum fuel flow which will be needed in use, and of course a safety margin is applied on top of that maximum. Thus there will be many operating conditions, notably engine idle conditions where the output of the pump exceeds the demand of the engine.
Excess fuel from the pump output is spilled back to the low pressure side of the fuel system through a spill valve which is arranged to operate to maintain a substantially constant pressure drop across the fuel metering valve of the fuel metering system. A pressure raising and shut-off valve (PRSOV) is interposed between the metering valve and the engine burners, and ensures that the fuel system upstream of the PRSOV is pressurised to a sufficient level that ancillary equipment powered by fuel pressure, for example engine control vanes, can be operated. Additionally the PRSOV provides a means of isolating the gas turbine engine burners from the fuel supply system when the engine is to be shut-off.
A recognised difficulty of the conventional system described briefly above is known as “heat rejection”. When the gear pump is pumping fuel against system pressure then the action of spilling fuel in excess of engine demand to the low pressure side of the system results in heating of the fuel. As fuel is used as a cooling medium for other engine system, the greater the level of rejection of heat into the fuel by the fuel pumps the lower is the capacity of the feel to cool other components. In addition, energy from the engine is wasted in heating the fuel through heat rejection at the fuel pumps thus resulting in an overall increase in engine fuel consumption.
It has been proposed to minimise heat rejection problems by utilising two gear pumps to supply fuel through the metering system to the engine, both pumps being driven simultaneously and continuously from the engine shaft, but at least one of the pumps being arranged to spill its output back to the low pressure side of the fuel system when its output is not needed the output being spilled directly back to the low pressure side of the system so that the pressure increase across the pump is minimal, and thus the heating of the fuel displaced by the pump is minimal. U.S. Pat. No. 4,245,964 discloses such a fuel supply system.
It will be recalled from the description of the conventional system above hat a PRSOV maintains system pressure upstream of the PRSOV at a predetermined minimum level consistent with safe operation of ancillary equipment of the engine. If the pressure in the system upstream of the PRSOV drops below the predetermined level then the engine is isolated by the PRSOV from the fuel supply system. During normal operation of a gas turbine engine the pressure upstream the PRSOV will be more than adequate, and the PRSOV will not isolate the engine from the fuel supply system unless specifically commanded to do so in an engine shut-down sequence.
In the event of combustion failure within an engine during flight (a, so called, “flame-out” condition) it is desirable to be able to relight the engine during flight by utilising the rotation of the engine shaft caused by the passage of air through the engine in flight (“windmilling”). Modern turbofan engines have a “windmill relight speed” which is very low compared with the normal shaft speed of the engine when operating in flight or when being started on the ground using either ground power or an air starter motor. The “windmill relight speed” is the engine shaft speed at which relight of a failed engine can be initiated when the engine shaft is being rotated by the passage of air through the failed engine in flight.
Operation of the PRSOV to maintain pressure upstream of the PRSOV can cause difficulties during windmill relight in that the rotational speed of the engine shaft, and thus the rotational speed and output of the gear pump is insufficient to provide the pressure rise in the fuel metering system necessary to open the PRSOV for fuel to be supplied to the engine to allow the engine to start (relight). Internal leakage within the pump, and parasitic flows in the fuel system can exacerbate the problem, but even when such internal leakage and parasitic flows are minimised the pump still may not be capable of delivering sufficient pressure rise into the system to open the PRSOV at windmill relight speeds. Our U.S. Pat. No. 6,176,076 discloses ways of minimising the pressure rise which is needed in the system in order for the PRSOV to be opened so that the engine can be started.
The solution proposed in our U.S. Pat. No. 6,176,076 is to provide a passive restrictor in the spill line from the gear pump, and to use the pressure increase in the spill-line provided by the passive restrictor to influence the pressure at which the PRSOV opens In such a system the pressure required to open the PRSOV varies in response to the amount of fuel spilled by the spill valve associated with the gear pump Thus in normal flight conditions, where a significant volume of fuel is being spilled from the gear pump output, the pressure developed across the passive restrictor will be relatively high resulting in a high pressure in the spill line which will cause the PRSOV to maintain a relatively high pressure upstream thereof in the metering system, whereas in engine start situations particularly windmill relight situations where very little, if any, fuel is spilled from the pump output due to the low pump speed the PRSOV is influenced by a much lower pressure developed across the passive restrictor in the spill line to maintain the pressure upstream of the PRSOV at a minimum value consistent with engine starting and allow the PRSOV to open at such lower pressure. Once the engine is started the pump speed and output volume will rise rapidly and the system pressure will increase as a greater volume of feel is spilled back to low pressure through the passive restrictor.
The use of such a passive restrictor in the spill line as disclosed in our U.S. Pat. No. 6,176,076 is a solution to the windmill relight difficulties of an engine supplied from a traditional, single, gear pump fuel supply system. However, use of such a passive restrictor would negate the heat rejection advantages of using twin gear pumps rather than a single gear pump as the gear pump whose output is not needed and which is being spilled back to the low pressure side, would then be pumping fuel against a pressure rise caused by the passive restrictor, and heating of the spilled fuel would result.
It is an object of the present invention to provide fuel supply systems utilising twin gear pumps in which the windmill relight problems can be minimised or obviated, without negating the heat rejection advantages of using twin gear pumps.